1. Technical Field
The present disclosure relates to a method and apparatus for testing the operating conditions of an electric network and, more particularly but without limitation, the operating conditions of a load of an electric network using changes in impedance or admittance.
2. Description of the Related Art
As used hereinafter, the term “operating conditions” is intended to relate not only to the operating condition of a load, but also to the condition in which some load components are short circuited or the condition in which some load components are open circuited.
Shorted or open operating states are known to occur in an electric network, e.g. when connections are established between two points of the circuit that impose a zero voltage at its ends, or when one of the circuit components is disconnected respectively.
In prior designs, the operating state of an electric network is tested by providing a voltage (or current) test signal to the load of the electric network, measuring the instantaneous current signal circulating in the load (or measuring the voltage at the ends of the load) and checking if the measured current (or voltage) value falls within a range of preset values, typically provided by the manufacturer.
If the measured current value is higher than the preset value, then the operating state of the electric network is deemed to be in a shorted condition, whereas if the measured current value is lower than the preset value, then the operating state of the electric network is deemed to be in an open condition.
Electric networks are also known to be affected by noise, which is defined as the totality of all undesired voltage or current signals overlapping the useful signal. In the above prior testing arrangements, such noise is a frequent source of errors, susceptible of causing inaccurate detection of the operating conditions of the electric network being tested.
In an attempt to reduce the risk of wrong detection caused by inaccurate measurement due to noise overlapping the useful signal, certain prior testing methods have used noise type-specific detection. For instance, the measuring step is designed to be repeated several times for the result to only be deemed valid when determined as such at all detections. Otherwise, the measuring test is deemed to be valid if the measured value remains stable for a predetermined time.
Further details about a practical example in which the operating conditions of the load are tested against actually expected conditions are illustrated in FIG. 1, which shows an audio amplifier 1 having an electric network 2 connected thereto, the latter including a two-way loudspeaker system composed of a woofer 2A and a tweeter 2B. The woofer 2A is connected to the amplifier 1 via an inductor 3, which acts as a low-pass filter, whereas the tweeter 2B is connected to the amplifier 1 via a high-pass filter, i.e., a capacitor 4. According to the prior design, voltage v(t) and current i(t) values flowing into the load 2 can be measured and stored for a given measurement time Tm.
In the circuit of FIG. 1, possible abnormal operating states to be detected include, without limitation:                open condition of the woofer 2A;        shorted condition of the woofer 2A;        open condition of the tweeter 2B; and        shorted condition of the tweeter 2B.        
Still according to the prior design, in order to determine whether the woofer 2A or the tweeter 2B or both are in short circuit and/or open circuit conditions, a comparison step is carried out using preset value ranges provided by the manufacturer of the electric network. Bearing in mind, for example, that when the impedance of the woofer 2A has to be measured, the test signal used for testing is generally applied to the electric network 2 in the form of a voltage (or current) slowly varying within a time interval much longer than its resonance frequency period. In the presence of noise also, such as a mechanical stress, an EMF is generated, which is typically a damped oscillation at the resonance frequency with an average value that can be considered zero.
FIG. 2 shows the curve of current i(t) in the electric network 2 of FIG. 1 when such mechanical stress EMF is introduced. It can be seen that the voltage v(t) (which is represented by a voltage step) imposed on the electric network 2 is added with a damped oscillation associated with the mechanical stress EMF.
In prior testing methods, the operating condition of the electric circuit is evaluated considering current amplitude i(t) only. Particularly, in the case of a DC load (woofer), the current level i(t) is checked to determine whether it falls between the specification limits indicated by the lines Ith(min) and Ith(max) as shown in FIG. 2. If the measured current value i(t) exceeds the threshold Ith(max), then the load L represented by the electric network 2 is deemed to be a short circuit. However, if the measured current value i(t) is below the threshold Ith(min), then the load represented by the electric network 2 is deemed to be an open circuit.
In order to reduce the risk of false detections caused by the current noise introduced by the mechanical stress EFM, either of the following actions may be taken:                the measuring step is repeated N times, and the result is only deemed valid when determined as such at all detections, or        the result of the measuring step is deemed valid if it is maintained throughout the measurement time Tm (e.g., 200 msec in the case of a woofer). Otherwise, the test is deemed to be invalid.        
Referring now to FIGS. 3A to 3C, waveforms are shown in which the current signal i(t) circulating in the tweeter 2B (FIG. 3A) at a 20 kH frequency is added with a noise signal n(t) (FIG. 3B) generated by an electromagnetic field or by the amplifier 1 itself. In the case of the electric network 2 of FIG. 1, prior testing includes determination of the operating condition by considering the number of above threshold occurrences of the measurement signal i(t) with the noise superposed thereto, with respect to a predetermined threshold H in a predetermined time Tm (FIG. 3C).
If the above-threshold H occurrences of the current i(t) in the measurement time Tm are greater than a predetermined number N of above-threshold occurrences, the load L is deemed to be present, whereas if such threshold H is never exceeded, the load L is deemed to be an open circuit. Also, if the number of above-threshold occurrences is greater than zero but smaller than N, the test is deemed to be invalid.
It shall be noted that the above testing embodiments relate to AC powered loads, and can only concern the tweeter 2B of the electric network 2. It shall be also noted that the shorted or open conditions as detected above do not cause considerable percentage changes in the equivalent modulus of impedance at the ends of the load provided by the electric network 2.
Particularly, with further reference to FIG. 4, it can be seen that, at 20 kH frequency, there is very little difference between the equivalent modulus of impedance of the electric network 2 in normal conditions and the modulus of impedance of the same electric network 2 when, for example, the tweeter 2B is shorted, which difference cannot be discriminated (see point indicated by the arrow in FIG. 4).
However, in FIG. 5, the modulus of impedance of the cases described above with reference to FIG. 4 exhibits an important phase difference, particularly 90 degrees (see point indicated by the arrow). Also, it can be seen from FIG. 6 that, at 20 kH frequency, there is very little difference between the equivalent modulus of impedance of the electric network 2 in normal conditions and the modulus of impedance of the same electric network 2 when, for example, the tweeter 2B is disconnected (open condition), which difference cannot be discriminated (see point indicated by the arrow). However, in FIG. 7, the modulus of impedance of the cases described above with reference to FIG. 6 exhibits a 60 degree phase difference (see point indicated by the arrow).
In view of the above, the embodiments of the present disclosure overcome the above mentioned prior art drawbacks.